The present invention is concerned with a process of producing transfer printing papers.
Transfer printing papers are employed for thermally applying colored images, in particular, varicolored pictures, letters and/or decorations on items retaining their shape at high firing temperatures, in particular, of ceramics or glass, in which the printable ceramic coloring pigments in a release composition are transferred to the transfer printing paper in the rotogravure process and are kept in homogeneous distribution in the release composition before their transfer to the rotogravure cylinder. A process of this type is taught by DE-A-42 03 162. This process was developed after it had been found that the quality of the transfer printing papers obtained in usual rotogravure processes was unsatisfactory. Compared thereto, the process according to DE-A-42 03 162 employing commercially available ceramic coloring pigments resulted in substantially improved replicas of the respective master. However, in the meanwhile, it has been found that no consistently uniform, i.e. reproducible duplication brilliance, is obtainable for all transfer papers printed in one batch, quite apart from the fact that the oversized cup depth deemed imperative in the process according to DE-A42 03 162 to enable a large amount of printing ink to be admitted, involves unsatisfactory results inasmuch as no consistently good printed products and, in consequence, no consistently good burning results are achievable.
The cup depth in conventional rotogravure printing, depending on the type of manufacture of the cups (etching or gravure), is between 2 xcexcm or 7 xcexcm, respectively, and about 60 xcexcm, with light areas of the master being taken into account by cups of a lower depth and dark areas by correspondingly deeper cups.
Hereinafter the following terminology will be used conveying the following meaning:
color pigments: dye particles with no glass addition;
ceramic color pigments: dye particles to which glass is added;
binder: the proportion in the printable ink composition binding the pigment and ceramic color pigments and being adjusted by organic solvents for rotagravure to the proper viscosity:
ink quantity: amount of the printable ink composition introducible into the cups.
Starting from the process of manufacturing transfer printing papers according to DE-A-42 03 162, it is the object of the invention to improve this process to safeguard the reproducibility of the images during the firing or heating of the items, and their optimally brilliant quality.
This problem is solved by the process of the invention in that, for printing the transfer paper, cup depths of between 20 xcexcm and 60 xcexcm are used, depending on the tinting of the master and the ceramic coloring pigments whose glass particle proportion, at least in the non-red pigments, is reduced as against the previously usual proportion in the substrate.
Hence, according to the present invention selected cup depths are used which are essentially in the range of between 20 and 60 xcexcm, preferably between 30 and 60 xcexcm which, as compared to DE-A-42 03 162, also results in a correspondingly reduced amount of ink introducible into the cups, since cup depths of up to 100 xcexcm are deemed necessary according to the prior art suggestion.
However, the specific selection of cup depths alone will not yet solve the problem encountered. Ceramic colouring pigments have, depending on the manufacturer of the pigments, a relatively high glass particle proportion in the order to about 70 to 80%, it being impossible to precisely define the association of the glass particles to the coloring pigments, i.e. to define whether, in part, they are agglomerated or, in part enclose the pigments entirely or only partially. It can only be assumed that, in particular, the high glass proportion and, possibly, the association or connection thereof to the pigments are responsible for the lack of a safe reproducibility of the images on the fired or heated items, especially so as DE-A-42 03 162 teaches that large amounts of ceramic coloring pigments had to be applied to the transfer printing paper through cups of great depth so that along therewith correspondingly large amounts of glass particles and, in particular, of binder (criticality of which, in the meanwhile, has been identified) were introduced into the print. Only for the sake of completeness it is pointed out that also in the present instance printing is effected on a transfer paper coated with a gelatine, wax or gum arabic layer, with the final print being provided with a socalled xe2x80x9ctransfer lacquerxe2x80x9d coating.
Hence, starting from the process according to DE-A-42 03 162, on the one hand, it is of decisive importance for a reproducible duplication brilliance of the master on the final product, to use cup depths commonly employed in rotagravure processes by focusing on a specific range of cup depths and, on the other hand, to reduce the glass proportion, i.e. the glass particle share of the ceramic coloring pigments must be correspondingly lower than that of commercially available ceramic coloring pigments.
The reduction of the glass particle share is easily reachedxe2x80x94if pigment manufacturers are not able to directly supply ceramic coloring pigments of a correspondingly reduced glass particle share as provided for according to the inventionxe2x80x94in that pure coloring pigments, i.e. pigments containing no glass, are added in corresponding dosages to the commercially available ceramic coloring pigments having high glass proportions, so that the glass proportion of an ink substrate will contain a correspondingly reduced amount of glass. Apart therefrom, the transfer printing paper manufactured according to the process of the invention not only does contain a correspondingly reduced amount of glass but also contains less binder in each of the cups and larger amounts of coloring pigments instead becausexe2x80x94as compared with the process according to DE-A-42 03 162xe2x80x94due to the lower cup depth a correspondingly lower amount of ink is introduced. As, on the one hand, cup depths, essentially, are in the afore-mentioned range and, on the other hand, color tints will have to be taken into account, this will be preferably achieved by corresponding cross-sectional dimensions (in plan view) of the cups and by the spacing at which they are arranged, as opposed to conventional rotagravure processes according to which, as a rule, this is achieved only by cup depths differing for the individual inks, with consistently equal-sized cup openings in the range of about 2 xcexcm to about 60 xcexcm being used. The xe2x80x9cat least the non-red coloring pigmentsxe2x80x9d restriction as referred to is based on that it has been found that, during firing, it is imperative for the high glass share to be present in red ceramic coloring pigments in order to maintain the red colour effect during firing. Accordingly, commercially available red ceramic coloring pigments containing a relatively high proportion can be readily used for the color red; however, this does not exclude a reduction in the glass proportion even in the red colour should the glass share, depending on the manufacturer, be particularly high.
Hence, the process of the inventionxe2x80x94compared to the process according to DE-A-42 03 162xe2x80x94insures, when using cup depths in the indicated range that, with a correspondingly reduced glass proportion in the substrate, a larger amount of coloring pigments can be introduced into each of the cups, this being not effected through enhanced cup depths but rather through an increased share of the coloring pigment in the respective dye substrate. Since, moreover, lower cup depth are used than in the process of DE-A-42 03 162, each of the cups contains less binder.
In respect of hard porcelain it has proved advantageous to overprint at least one printed ink layer or at least the uppermost layer among a plurality of ink layers with a final layer of glass particles, to thereby quasi restore the reduced glass proportion so that, during firing, each of the pigment particles is safely bonded in the glass. Glass particles sized about 3 to 5 xcexcm are used for cover layers of this type, with the glass particle layer being applied in a thickness of, for example, up to between 20 and 30 xcexcm which can, for example, also be done by printing. The ratio of solvent and binder to ceramic coloring pigments suggested by DE-A-42 03 162 is not substantially changed, although in the present instance less glass and, in particular, also less binder, are contained in the printable ink substrate. In this respect, reference is made to the following embodiment.
The transfer printing papers manufactured according to the process of the invention are especially suitable for use with ceramics, china and glass and, optionally, also with metallic items in respect of which the transfer papers manufactured according to the invention not only do involve the advantage that objects, such as tiles, in an extremely low-cost way, can be decorated with fanciful designs but also do involve the option to replace, in a non-polluting way, bottle labels of paper by burnt-in designs of whatever fanciful character, with the latter, in general, being even less costly than paper labels in respect of which disposal must be provided for after use and removal thereof.
If due to particularly pale areas of the master the cup depths need not be less than 30 xcexcm, the process of the invention preferably employs printing blocks respectively having an invariable cup depth for each ink with the different inking in print being, as mentioned before, achieved by the spacings at which the cups are arranged and/or by cross-sectional dimensions varied in accordance with the master.
The transfer printing papers manufactured according to the process of the invention also can be used for soft porcelains (bone china). However, since soft porcelains, as mentioned hereinbefore, are provided with a glaze it will in that case not be required to coat the transfer papers with a thin layer of glass particlesxe2x80x94as is done, with advantage, in transfer papers for hard porcelainxe2x80x94for, the print transferred to the glaze by a transfer lacquer, i.e. the colouring pigments and ceramic pigments, during firing, are adequately safely melted into the glaze.
The particle size of the color pigments in the process of the invention is approximately  less than 10 xcexcm, preferably  less than 5 xcexcm. In connection with the process of the invention it has, moreover, proved to be advantageous for achieving, in printing, an optimum transition of the dye from the cup to the transfer paper, to use non-undercut cups of the type known per se in traditional rotogravure processes. The specific use of this type of cups completely open toward the printing side according to another embodiment of the process of the invention, in the present instance, is of a special significance as it is of primary importance to completely evacuate the cups during printing and to transfer the contents thereof to the transfer printing paper.
The reason for indicating only ranges and approximate figures for the cup depths and proportions is that ceramic coloring pigments supplied by different manufacturers always are differing from one another to a certain degree. However, on the one hand, always the reduction of the glass proportion in conjunction with an increased share of the coloring pigment and, on the other hand, the pointed resort to cup depths commonly used in traditional rotogravure remain of decisive importance, with the latter involving again a lower amount of binder in the cups and, hence, in the transfer printing paper.
If suppliers or manufacturers are unable to indicate or to reliably indicate the glass proportion contained in the ceramic coloring pigments, it will be necessary for the manufacturer of transfer printing papers to determine the glass share in order to be able to add, on the basis thereof, the required amount of coloring pigments.